1. Field of Invention
The present invention relates generally to reflective polarizing films where the reflectivity may be varied by impressing an electric field across the film. Such films may be used to great advantage in electro-optical glazing structures having reflection, semi-transparent, and/or transparent modes of operation which are electrically-switchable for use in dynamically controlling electromagnetic radiation flow in diverse applications. In particular, the present invention relates to reflective polarizing films where the bandwidth of the reflectivity may be controlled by impressing an electric field across the film.
2. Brief Description of the Literature
Broadband reflecting polarizers were introduced in copending application Ser. No. 08/550,022 (now U.S. Pat. No. 5,691,789) entitled xe2x80x9cSingle Layer Reflective Super Broadband Circular Polarizer and Method of Fabrication Thereforxe2x80x9d by Sadeg M. Faris and Le Li which was filed Oct. 30, 1995. Such broadband polarizers are made by producing a single layer having cholesteric liquid crystal order where the pitch of the liquid crystal order varies in a non linear fashion across the layer. A narrow band, switchable polarizing single layer reflector is disclosed in European patent application 0 643 121 A, published Mar. 15, 1995. A switchable polarizing single layer reflector having a broader bandwidth is disclosed in PCT application WO97/2358, published Jul. 3, 1997. General references on polymer dispersed liquid crystals may be found in detail in xe2x80x9cPolymer Dispersed Liquid crystal displaysxe2x80x9d, by J. W. Doane, a chapter in xe2x80x9cLiquid Crystalsxe2x80x9d, Ed. B. Bahadur, World Scientific Publishing, Singapore, and xe2x80x9cCLC/polymer dispersion for haze-free light shuttersxe2x80x9d, by D. Yang et al. Appl. Phys. Lett. 60, 3102 (1992). Smart Window Design is treated in xe2x80x9cElectrochromism and smart window designxe2x80x9d, by C. Granqvist, Solid State Ionics 53-56 (1992) and xe2x80x9clarge scale electochromic devices for smart windows and absorbersxe2x80x9d, by T. Meisel and R. Baraun, SPIE 1728, 200 (1992). The above identified US patents and other references are hereby incorporated by reference.
It is an object of the invention to provide a reflective polarizing film having a bandwidth which is controllable by an electric field.
It is an object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth which is controllable by an electric field.
The present invention provides a single layer polarizing reflective film comprising a crosslinked polymer matrix mixed with low molecular weight liquid crystal molecules. The low molecular weight liquid crystal molecules are oriented with respect to the surface of the film and to each other in cholesteric order. The ratio of the amount of liquid crystal molecules to the amount of cross-linked polymer is chosen so that the liquid crystal molecules may move reversibly in an electric field. The movement of the low molecular weight molecules perturbs the cholesteric liquid crystalline order responsible for the reflectivity of the polarized light. If the composition of the film is uniform, the polarized reflectivity of the film has a very narrow bandwidth when there is no electric field impressed in the film. As the electric field is increased, the bandwidth of the polarized reflectivity increases. There is sufficient high molecular weight cross linked polymer material to ensure that the film is not liquid, and to ensure that the low molecular weight material does not diffuse after manufacture of the film, and to ensure that the low molecular weight material returns to the cholesteric liquid crystal ordered state when the field is removed.